1. Field of the Invention
The present invention relates to communication networks and more specifically to network switches and associated switch to switch protocols which provide improved bandwidth utilization and load balancing in data processing communication networks having redundant paths between network-devices.
2. Related Patents
This patent is related to the following commonly owned patents: U.S. patent Ser. No. 09/228,110 entitled Load Balancing Switch Protocols, U.S. patent Ser. No. 09/228,159 entitled Identity Negotiation Switch Protocols, U.S. patent Ser. No. 09/228,890 entitled Cost Propagation Switch Protocols, U.S. patent Ser. No. 09/228,913 entitled Cost Calculation in Load Balancing Switch Protocols, U.S. patent Ser. No. 09/228,918 entitled MAC Address Learning and Propagation in Load Balancing Switch Protocols, U.S. patent Ser. No. 09/228,992 entitled Path Recovery on Failure in Load Balancing Switch Protocols, and U.S. patent Ser. No. 09/228,169 entitled Discovery of Unknown MAC Addresses Using Load Balancing Switch Protocols, all of which are hereby incorporated by reference.
3. Discussion of Related Art
It is common in present computing environments to connect a plurality of computing systems and devices through a communication medium often referred to as a network. Such networks among communicating devices permit devices (or users of devices) to easily exchange and share information among the various devices. The Internet is a presently popular example of such networking on a global scale. Individual users attach their computers to the Internet, thereby enabling sharing of vast quantities of data on other computers geographically dispersed throughout the world.
Networked computing systems may be configured and graphically depicted in a wide variety of common topologies. In other words, the particular configurations of network communication links (also referred to as paths) and devices between a particular pair of devices wishing to exchange information may be widely varied. Any particular connection between two computers attached to a network may be direct or may pass through a large number of intermediate devices in the network. In addition, there may be a plurality of alternative paths through the network connecting any two network devices. Present day computing networks are therefore complex and vary in their configurations and topologies.
Most present network communication media and protocols are referred to as packet oriented. A protocol or communication medium may be said to be packet oriented in that information to be exchanged over the network is broken into discrete sized packets of information. A block of information to be transferred over the network is decomposed into one or more packets for purposes of transmission over the network. At the receiving end of the network transmission, the packets are reassembled into the original block of data.
In general, each packet includes embedded control and addressing information that identifies the source device which originated the transmission of the packet and which identifies the destination device to which the packet is transmitted. Identification of source and destination devices is by means of an address associated with each device. An address is an identifier which is unique within the particular computing network to identify each device associated with the network. Such addresses may be unique to only a particular network environment (i.e., a network used to interconnect a single, self-contained computing environment) or may be generated and assigned to devices so as to be globally unique in co-operation with networking standards organizations.
At the lowest level of network communication, such addresses are often referred to as MAC address (Media ACcess address). Network protocols operable above this lowest level of communication may use other addresses for other purposes in the higher level communication techniques. But in most network low level communication levels, operable on the physical link medium, an address is referred to as a MAC address.
In many present commercially available network environments, the network communication medium is in essence a bus commonly attached to a plurality of devices over which the devices exchange. In a simple networking topology, all devices may be attached to a such a bus structured common network medium. Any particular single network medium has a maximum data exchange bandwidth associated therewith. The maximum data exchange bandwidth of a medium is determined by a number of electrical and physical properties of the medium and protocols used to communicate over that medium. For example, a popular family of related network media and protocols are collectively referred to as Ethernet. Ethernet defines a standard protocol for the exchange of messages over the communication medium. A variety of communication media are also defined as part of the Ethernet family. The communication bandwidth of the Ethernet family of standards range from approximately 10 Mbit (million bits of information) per second to 1 Gbit per second. Therefore, a single (slow) Ethernet connection, for example, has a maximum data exchange bandwidth of approximately 10 Mbit per second.
In present network computing environments, a number of devices are used in addition to interconnected computing systems to efficiently transfer data over the network. Routers and switches are in general network devices which segregate information flows over various segments of a computer network. A segment, as used herein, is any subset of the network computing environment including devices and their respective interconnecting communication links. As noted above, a single computer network communication link has a maximum data transfer bandwidth parameter defining the maximum rate of information exchange over that network. Where all devices on a computer network share a common network medium, the maximum bandwidth of the computer network may be rapidly reached. The overall performance of the networked computing environment may be thereby reduced because information exchange requests may have to await completion of earlier information exchange requests presently utilizing the communication link.
It is often the case, however, that particular subsets of devices attached to the network have requirements for voluminous communication among members of the same subset but less of a requirement for information exchange with other devices outside their own subset. Though standard switch features generally do not include identifying such logical groupings of devices, some enhanced switching features do permit such logic to be performed within a switch device. For example, some enhanced switch features include the concept of defining and routing information based on virtual LAN (VLAN) definitions. In a VLAN, a group of devices may be defined as logically being isolated on a separate network although physically they are connected to a larger network of devices. VLAN features of enhanced switches are capable of recognizing such VLAN information and can route information appropriately so that devices in a particular VLAN are logically segregated from devices outside the VLAN.
For example, the financial department of a large corporation may have significant information exchange requirements within the financial department but comparatively insignificant needs for data exchange with other departments. Likewise, an engineering group may have significant needs for data exchange within members (computing systems and devices) of the same engineering group but not outside the engineering group. There may in fact be multiple of such subsets of devices in a typical computing network. It is therefore desirable to segregate such subsets of devices from one another so as to reduce the volume of information exchange applied to the various segments of the computer network.
In particular, a switch device is a device that filters out packets on the network destined for devices outside a defined subset (segment) and forwards information directed between computing devices on different segments of a networked computing environment. The filtering and forwarding of such information is based on configuration information within the switch that describes the data packets to be filtered and forwarded in terms of source and/or destination address information (once address locations are “learned” by the switch(es)).
Network switch devices and protocols associated therewith are also used to manage redundant paths between network devices. Where there is but a single path connecting two network devices, that single path, including all intermediate devices between the source and destination devices, represent a single point of failure in network communications between that source and destination device. It is therefore common in network computing environments to utilize a plurality of redundant paths to enhance reliability of the network. Multiple paths between two devices enhances reliability of network communication between the devices by allowing for a redundant (backup) network path to be used between two devices when a primary path fails.
FIG. 1 shows an exemplary, simple networked computing environment in which multiple paths exist for communication between devices A 100, B 102, and C 104. These exemplary network devices are each attached to one of a plurality of switches (S1106, S2108, S3110, and S4112). Each device has multiple possible paths to each of the other two devices. For example, device A 100 may exchange information with device C 104 through any of three possible paths (via switches S1106 and S4112, respectively). The first exemplary path is a direct path connecting device A 100 directly to device C 104 through a port on switch S1106 and a port on switch S4112. A second path is through switch S1106 to switch S3110 and then through switch S4112. A third path is via switch S1106, switch S2108, and switch S4114. These three paths may be used as redundant communication paths connecting the two devices A 100 and C 104. Where a first path fails, the second path or third may be activated to assume responsibility for exchange of information between devices A and C. In like manner, there are three paths for communication between devices A 100 and B 102 and between devices B 102 and C 104.
Where redundant paths are available in such network computing environments, it remains a problem to effectively utilize the full available bandwidth. It would be desirable to utilize all redundant paths in parallel so as to increase the available communication bandwidth between two communicating devices. Where only a single path is used, the maximum bandwidth for exchange of information is limited to that of a single communication link. Where, on the other hand, all redundant links are used in parallel, the maximum communication bandwidth is increased by the number of links used in parallel. For example, as shown in FIG. 1, the communication bandwidth between any of the devices could, in theory, be increased by up to a factor of three.
However, as presently practiced in the art, protocols used among switch devices (e.g., S1106 through S4112) render such parallel communication paths unusable. Switches 105 through 112 as presently practiced in the art often use a protocol commonly referred to as “spanning tree” to discover the existence of redundant communication paths as known to a network of switches. The spanning tree protocol is described in detail in a proposed IEEE standard P802.1p entitled Standard for Local and Metropolitan Area Networks Supplement to Media Access Control (MAC) Bridges: Traffic Class Expediting and Dynamic Multicast Filtering. 
The spanning tree protocol as implemented in switches broadcasts (more precisely multicasts) information from the switch out to all devices that recognize the selected multicast address connected to paths from the switch. A multicast message is one which is directed to all devices rather than to a particular destination address on the network. The information in the multicast message describes the address forwarding information known to that switch. From such information shared among all the switches, each switch can derive the various paths in the network. Each switch device so attached to the multicasting device receives the information and forwards (multicasts) the message to each device attached to it (except the path from which it directly received the message), and so on. If such a multicast message returns on a path to the originating device, a loop must exist among the paths connecting the various switches. To reduce the number of messages generated on the network by virtue of such multicast messages, the spanning tree protocol requires that redundant paths so discovered be disabled. In a large network without spanning tree protocol to disable redundant paths, received multicast messages can “cascade” from each receiving switch to all other attached switches. The volume of such cascading messages may grow rapidly or even exponentially. Such multicast messages exchanged among the switched may in fact require a substantial portion of the available communication bandwidth of the network. Such conditions are often referred to as “broadcast storms.”
The spanning tree protocol therefore requires the disabling of redundant paths to avoid broadcast storms. Only when a path is known to have failed will a redundant path be enabled and used for the exchange of data. The spanning tree protocol therefore precludes aggregation of the available bandwidth to improve communication bandwidth by using multiple redundant paths in parallel. FIG. 2 is a block diagram of the same exemplary network of FIG. 1 where three communication links 114 between the switches have been disabled to prevent loops in the network and the resultant broadcast storm otherwise inherent in the spanning tree protocol.
Another problem with the spanning tree protocol arises from the fact that a preferred path may be unavailable due to the need to disable paths that cause loops among the switches. For example, as shown in FIG. 2, the preferred path between switches S1106 and S4112 may be the direct one which is disabled. To leave this direct communication link enabled would permit loops in the paths among the switches. Rather, a more circuitous route through switches S1, 106, S3110 and S4112 must be used to exchange information between switches S1106 and S4112. The spanning tree protocol does not assure that the best path between two switches will be left enabled. Rather, it merely attempts to assure that some path between switches is available, specifically, a relatively minimal path connecting all switches—a spanning tree.
The spanning tree protocol therefore precludes maximizing use of available bandwidth in a network of switches.
Some switches have provided a partial solution to this problem by using a technique known as “trunking.” Where there are multiple paths directly between two switches, the multiple paths serve as redundant communication paths but are trunked by the switches and treated logically as though they were a single path with higher maximum bandwidth. FIG. 3 is a block diagram of the same exemplary network environment of FIG. 2 wherein a plurality of communication paths between switch S1106 and S3110 are trunked. The communication path between switches S1106 and S3110 is therefore capable of using the trunked paths between them as though they were a single connection in terms of the spanning tree protocols. Since the redundant paths are treated as a single path for purposes of the spanning tree protocols, they need not be shut down to preclude broadcast storms,
However, trunking does not address the bandwidth issue in a broad sense. Rather, the trunking technique is only applicable where the multiple paths are between a particular pair of switches. The bandwidth limit is merely shifted from that of a single communication link to that of the number of communication links supported by a single switch.
It is a further problem that by precluding use of redundant links between switches, the spanning tree protocol also precludes the ability to balance communication loads among the redundant paths between switches. Where such multiple paths are allowed to simultaneously operate, it would be desirable for the switches to distribute the packet exchange communication among them over such multiple paths. Such distribution, often referred to as load balancing, further enhances the ability of the network to optimize the utilization of available throughput in the network of switches. Underutilized paths may be used to offload packet communication on overloaded paths.
It is therefore a problem in present networks of switches to simultaneously operate redundant paths between switches of the network to thereby maximize utilization of available bandwidth and to thereby communicate among the switches to balance communication loads over redundant paths.
A particular problem arises in present network switches using the spanning tree protocol in that the time required for broadcast messages to propagate through network switches will be maximized for a particular pair of devices. As noted above, broadcast messages are those which are generated by a device in the network to be transmitted to all other devices in the network. The broadcast messages are said to be propagated in that the message will travel through each switch in the single active path to be forwarded to all devices connected to the switches. The message will be sent from an initiating switch to its neighbor. Each neighbor will send in to its neighbor, and so on in sequence until each switch in the single active path has received the message for forwarding to its attached devices.
Under spanning tree protocol, there is but a single active path through the network switches to interconnect all devices. The single path is not intended to be an optimally minimal path. Rather, it is a path guaranteed to connect switches so as to attach all devices to the network. The single path may be thought of as a daisy chain of switches and specifically assures that devices connected to the switches at each end of the single active path will require a maximum time for propagation of broadcast messages. Specifically, if a device connected to the switch at one end of the single active path generates a broadcast message, it must of necessity pass through all other switches in the network to reach the device connected to the switch at the opposite end of the single active path. Broadcast messages generated by other intermediate network switches in the single active path initiate transmission of the message to each of its two neighbors so that the message is propagating simultaneously down two directions of the single active path. This may require less time to propagate as compared to the maximum time, but the minimum broadcast propagation time required will be at least the time required to propagate through one half of the switches in the single active path.
At best it may be said that broadcast propagation time in a network of switches operating with the spanning tree protocol is variable and less than optimal. The time is variable in that it depends on the position in the active path of the switch which initiates propagation of the broadcast message. It is therefore desirable to both reduce the broadcast propagation time required to reach all devices (i.e., the maximum broadcast propagation time) as well as to reduce the deviation in broadcast propagation times.